Successful Stimulation in the Cotton Valley Sandstone - A Low-Permeability Reservoir

Abstract

Thorough evaluation of field results in the Cotton Valley sand resulted in a dependable, successful approach to stimulation. This paper presents a history of the development of this approach and verifies successes with case histories of Cotton Valley wells. Introduction The Cotton Valley sandstone, located in north-central Louisiana and East Texas, is an extensive, coastal strand-plain sandstone deposition. Formation permeability and porosity are very low and stimulation is necessary permeability and porosity are very low and stimulation is necessary in most Cotton Valley wells to obtain economic production. Because of the depth of the formation (8,000 to production. Because of the depth of the formation (8,000 to 12,000 ft) and the reservoir temperature encountered (230 to 280 deg. F), the choice of fracturing procedures is somewhat limited. During the past 3 years, much success has been obtained with hydraulic fracturing using moderately viscous, temperature-stable, aqueous gel systems. Recently, in wells where fracturing-fluid retention has been a problem, the use of low-surface-tension surfactants and problem, the use of low-surface-tension surfactants and carbon dioxide with the fracturing fluid has helped improve fracturing-fluid recovery after treatment and has provided encouraging stimulation results. provided encouraging stimulation results.After a thorough evaluation of field results in the Cotton Valley sand, a dependable, successful treatment has been developed. This paper provides a history of the development of this approach and verifies present successes with case histories of Cotton Valley wells. Geologic Description Since drilling activity began on a large scale in north-central Louisiana in the early 1950's, the presence of an extensive group of sand bodies called the Cotton Valley Group has been known. The geological deposition of the area became better defined as well-control information was obtained. As early as 1944, Swain was able to describe geologically the over-all deposition history of the group of Upper Jurassic sands known as Cotton Valley. Swain also described Upper Jurassic deposition in northeastern Texas. In 1954, Forgotson further described these sediments and added some changes to Swain's descriptions. More recently, Thomas and Mann further defined the depositional trend of the Cotton Valley. This description is still believed to be fairly accurate. A general map of the Cotton Valley deposits and areas of interest is shown in Fig. 1. The sand member is 500 to 600 ft thick and grades into a thinner marine deposition, dipping to the southwest. The main sand body pinches out to the north with an east-west line in southern Arkansas and becomes lenticular in eastern Louisiana. During Late Jurassic time, the coast that bordered the Gulf of Mexico extended east and west across southern Arkansas. Parallel with the coast line, a long-shore barrier island in northern Louisiana separated a coastal lagoon in northernmost Louisiana from the open Gulf on the south. A delta that was formed by the ancestral Mississippi River in northeastern Louisiana and adjacent Mississippi supplied sand to the long-shore barrier island. The Late Jurassic coastal features are reflected clearly in the composition of the rock of the Cotton Valley Group. A well log showing "typical" Cotton Valley deposition is shown in Fig. 2. JPT P. 1267